1. Field of the Invention
The present invention relates generally to aluminum alloy brazing sheet materials including brazing sheet tube stock, and more particularly to erosion/corrosion resistant clad aluminum alloy brazing sheet materials as well as to methods for their manufacture and use.
2. Description of Related Art
In the recent past, automobile manufacturers have increasingly turned to employing aluminum radiators, heater cores, evaporators, condensers and other heat exchangers due to aluminum's superior properties, especially in terms of strength, durability, weight and corrosion resistance. Aluminum heat exchanger materials must meet several demanding properties in order to be acceptable for long term use in vehicles including good heat transfer, high strength to withstand internal pressures, corrosion resistance to cooling fluids on the interior of the units and external corrosion resistance to salt and other road chemicals.
Aluminum heat exchangers are typically formed by brazing together parts fabricated from clad aluminum sheet components in which one of the clad materials is a low melting aluminum alloy, generally made from 4xxx series aluminum.
The tube stock material is generally formed of brazing sheet material stock. The selection of the composition of the tube stock is typically made with different considerations in mind than those that are relevant in selecting the composition of clad materials since the inner clad portion of the tube is exposed to antifreeze, water and other fluids flowing through the interior of the tubes while the exterior of the tubes are exposed to the same conditions as the fins since they are positioned on the exterior of the radiator or heat exchanger. Braze clad fins have been formed, for example, from AA3003. See, i.e. Örtnäs et al. Sagging Resistance of Braze Clad Fin Material for Evaporators and Condensers SAE International Congress & Exposition, Detroit, Mich., No. 960246 pp. 11-16 (1996).
Brazing sheet materials for heat exchangers can be comprised of multiple layers. See, e.g. U.S. Pat. No. 5,292,595 that discloses a 3 layered clad material. Typically, the aluminum alloy brazing stock material comprises an aluminum core alloy that has been clad on one or both sides with other aluminum based alloys. If clad on both sides, the cladding alloys can be of different compositions and thicknesses. The composition of the core and the clad are carefully selected and are important to the properties of the resulting brazing sheet material. In the past, materials such as AA3005 and AA3003 (0.05% Cu, approximately 1% Mn, up to 0.6% Si and up to 0.7% Fe) have been used as the core alloy for such things as radiator tube stock and their inner surfaces have been covered with a clad layer of another aluminum alloy such as AA7072, for example. AA7072 is a lower strength alloy that includes approximately 1-1.5% Zn so that the inner liner acts to reduce corrosion of the core alloy through Galvanic protection.
However, substantial problems exist with corrosion of traditional tube stock core alloys from both the interior and exterior of the tube. Particularly, problems arise on the inside of the tubes due to erosion/corrosion of the clad layer which occurs due to the extremely high velocity of coolant that is moving through the tubes. Garcia et al. in “Internal Corrosion/Erosion Testing of Welded Aluminum Radiator Tubes for Passenger Cars and Heavy Duty Trucks,” IMechE, pp. 257-265 (1995) investigated the properties of the internal corrosion and erosion/corrosion associated with the interior of radiator tubes as a function of fluid flow.
Moreover, in recent years, the popularity of sport utility vehicles (“SUV's”), multi-passenger vehicles and heavy trucks has increased as well as additional options that consume engine power, which places heavier demands on the heat exchanger. In order to adequately heat/cool these larger scale vehicles, the vehicle manufacturers have merely increased flow rate of the coolant through the heat exchanger so as to provide more cooling/heating with the same size unit. This is because the size of the heat exchanger may already be at its maximum size, and as such, the only way to achieve adequate results is to increase the flow of fluid through the unit. Again, the increase in fluid flowing through the tubes will, in turn, change the dynamics of the interior of the tubes and will even erode the interior due to high pressures and flow rates over time.
It would be desirable to find a suitable brazing sheet material and associated method whereby the size of the heat exchanger can be minimized, while still achieving acceptable internal erosion/corrosion resistance properties.